Butadiene rubber particles with secondary particle sizes for epoxy resin encapsulant

ABSTRACT

An epoxy resin composition for semiconductor encapsulation comprising (A) an epoxy resin, (B) a phenolic resin, and (C) butadiene rubber particles having an average particle size of secondary particles of 100 μm or smaller and having such a size distribution that the proportion of secondary particles having a particle size of 250 μm or smaller is 97% by weight or more, and the proportion of secondary particles having a particle size of 150 μm or smaller is 80% by weight or more. Component (C) is uniformly dispersed in the composition without forming coarse agglomerates to secure low stress properties.

FIELD OF THE INVENTION

This invention relates to an epoxy resin composition for semiconductorencapsulation, which has low stress properties and high reliabilityagainst moisture, a process for producing the epoxy resin composition,and a semiconductor device encapsulated with the epoxy resincomposition.

BACKGROUND OF THE INVENTION

Semiconductor elements, such as transistors and IC chips, areencapsulated into ceramic or plastic packages to be supplied assemiconductor devices protected from the outer environment and easy tohandle. Ceramic packages have excellent moisture resistance because ofthe character of the ceramic material itself and impose little stress tothe semiconductor element because of their hollow structure. Ceramicpackages therefore achieve highly reliable encapsulation. However, theceramic materials are expensive, and the ceramic packages are lesspractical for mass production than plastic packages.

Therefore, plastic packages using an epoxy resin composition have beenleading recently. Plastic packages, while suitable for mass productionand less expensive, allow moisture to permeate and have a greater linearexpansion coefficient as compared with a semiconductor elementencapsulated. Therefore, it has been a weighty subject in the art toimprove moisture resistance and low stress properties.

The stress problem of encapsulating resins has been coped with bydispersing rubber particles, such as butadiene rubber particles, in theresin matrix. However, since rubber particles exhibit high cohesion,commercially available butadiene rubber particles usually have anaverage secondary particle diameter of about 100 to 500 μm. Such rubberparticles fail to be dispersed uniformly in the encapsulating resin,which has imposed a limitation on the improvement in low stressproperties required of a semiconductor encapsulation material.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an epoxy resincomposition for encapsulating semiconductors in which rubber particlesare uniformly dispersed, being prevented from agglomerating.

Another object of the invention is to provide a process for producingsuch an epoxy resin composition without involving complicated steps.

Still another object of the invention is to provide a semiconductordevice which is free from the problems of stress and moisturepenetration.

The inventors of the present invention have conducted extensive studyfor the purpose of obtaining an encapsulating resin that achievesreduction in stress and improvement in reliability against moistureattack without affecting the appearance. The inventors study has beenfocused on a means for uniformly dispersing butadiene rubber particlesin an epoxy resin composition while preventing the particles fromforming large secondary particles without involving complicated steps.Noting that butadiene rubber particles are liable to agglomerate, theinventors have conducted investigation chiefly into the average particlesize of secondary particles and the proportions of secondary particlesfalling in specific size ranges. As a result, they have succeeded inuniformly dispersing butadiene rubber particles in an epoxy resincomposition by using butadiene rubber particles having a specificaverage particle size and a specific particle size distribution ofsecondary particles.

The inventors have also found that reliability against moisture isfurther ensured by adding silicone oil having at least one amino groupper molecule together with the specific butadiene rubber particles. Thepresent invention has been completed based on these findings.

That is, the above objects of the invention are accomplished by;

(1) An epoxy resin composition comprising

(A) an epoxy resin,

(B) a phenolic resin, and

(C) butadiene rubber particles having an average particle size ofsecondary particles of 100 μm or smaller and having such a sizedistribution that the proportion of secondary particles having aparticle size of 250 μm or smaller is 97% by weight or more, and theproportion of secondary particles having a particle size of 150 μm orsmaller is 80% by weight or more;

(2) A semiconductor device comprising a semiconductor elementencapsulated in the above-described epoxy resin composition.

(3) A process for producing the above-described epoxy resin composition,comprising mixing at least components A, B and C described above; and

(4) A semiconductor device comprising a semiconductor elementencapsulated in an epoxy resin composition produced by theabove-described process.

Having the above-described specific size and size distribution, thebutadiene rubber particles used in the invention are uniformly dispersedin the matrix resin without forming coarse agglomerates, which can beachieved without involving complicated steps, thereby providing an epoxyresin composition for semiconductor encapsulation with ease and at lowcost. The semiconductor device sealed in the epoxy resin composition ofthe invention exhibits excellent low stress properties owing to theuniformly dispersed specific butadiene rubber particles. The epoxy resincomposition additionally comprising silicone oil having at least oneamino group per molecule exhibits further improved reliability againstmoisture.

DETAILED DESCRIPTION OF THE INVENTION

The epoxy resin composition according to the present invention comprises(A) an epoxy resin, (B) a phenolic resin, and (C) specific butadienerubber particles. It is usually supplied in the form of powder ortablets.

The epoxy resin as component (A) is not particularly limited andincludes various types of epoxy resins, such as a dicyclopentadienetype, a cresol novolak type, a phenol novolak type, bisphenol type, anda biphenyl type. These epoxy resins can be used either individually or amixture of two or more thereof. Of these epoxy resins preferred arethose having a melting point or softening point that is higher than roomtemperature. Generally useful epoxy resins include novolak epoxy resinshaving an epoxy equivalent of 150 to 250 and a softening point of 50 to130° C. and cresol novolak epoxy resins having an epoxy equivalent of180 to 210 and a softening point of 60 to 110° C.

The phenolic resin as component (B) acts as a curing agent for the epoxyresin. It is not particularly limited and includes dicyclopentadienetype phenolic resins, phenol novolak resins, cresol novolak resins, andphenol aralkyl resins. They can be used either individually or as amixture of two or more thereof. Phenolic resins having a hydroxylequivalent of 70 to 250 and a softening point of 50 to 110° C. arepreferably used.

The phenolic resin as component (B) is preferably used in such an amountthat the hydroxyl equivalent of the phenolic resin is 0.5 to 2.0,particularly 0.8 to 1.2, per epoxy equivalent of the epoxy resin ascomponent (A).

The butadiene rubber particles as component (C) are usually obtained bycopolymerization of butadiene and comonomers such as alkylmethacrylates, alkyl acrylates, and styrene. Examples of typicalbutadiene rubbers are a methyl acrylate-butadiene-styrene copolymer, amethyl acrylate-butadiene-vinyltoluene copolymer, a butadiene-styrenecopolymer, a methyl methacrylate-butadiene-styrene copolymer, methylmethacrylate-butadiene-vinyltoluene copolymer, a methylmethacrylate-ethyl acrylate-butadiene-styrene copolymer, abutadiene-vinyltoluene copolymer, and an acrylonitrile-butadienecopolymer. Preferred of them is a methyl methacrylate-butadiene-styrenecopolymer, particularly one having a butadiene content of 70% by weightor less and a methyl methacrylate content of 15% by weight or more. Anespecially preferred is a methyl methacrylate-butadiene-styrenecopolymer having a butadiene content of 40 to 70% by weight and a totalcontent of methyl methacrylate and styrene of 30 to 60% by weight. Theweight ratio of styrene to methyl methacrylate is preferably 0.5 to 2.0.A methyl methacrylate-butadiene-styrene copolymer prepared bypulverizing a commercially available Metaburene (a powdered product ofMitsubishi Rayon Co., Ltd.) by freeze grinding, etc. can also be used ascomponent (C).

It is also preferred for the butadiene rubber particles to have acore-shell structure. The butadiene rubber particles having a core-shellstructure are composed of a core of a butadiene rubber and an outershell of a polymer resin. The butadiene rubber making the core includesa styrene-butadiene copolymer latex and an acrylonitrile-butadienecopolymer latex, and the polymer resin making the outer shell includes apolymer resin having a glass transition temperature of 70° C. or higher.The polymer resin is obtained by polymerizing an unsaturated monomerhaving an unsaturated double bond, such as methyl methacrylate,acrylonitrile, and styrene. Such a core-shell structure can be formed bywater-mediated polymerization. In detail, a butadiene rubber is mixedwith water and polymerized to form butadiene rubber particles. Anunsaturated monomer is then added to the water medium and graftcopolymerized to the surface of the butadiene rubber particles (cores)to form an outer shell. Such core-shell butadiene rubber particlespreferably include those composed of a core made of a styrene-butadienecopolymer core and an outer shell made of methyl methacrylate or amethyl methacrylate-styrene copolymer. They preferably have a butadienecontent of 70% by weight or less and a methyl methacrylate content of15% by weight or more. They still preferably have a butadiene content of40 to 70% by weight and a total content of methyl methacrylate andstyrene of 30 to 60% by weight at a styrene to methyl methacrylateweight ratio of 0.5 to 2.0.

The butadiene rubber particles as component (C) should have an averageparticle size of its secondary particles of 100 μm or smaller. Apreferred average particle size of the secondary particles is 20 to 100μm, particularly 20 to 50 μm. The average particle size od secondaryparticles falling within a range of from 20 to 100 μm secures handlingproperties and dispersibility of the powder.

The butadiene rubber particles as component (C) should have such a sizedistribution that the proportion of secondary particles having aparticle size of 250 μm or smaller is 97% by weight or more, and theproportion of secondary particles having a particle size of 150 μm orsmaller is 80% by weight or more. It is preferred that the maximumparticle size of secondary particles be 250 μm or smaller, and theproportion of secondary particles having a particle size of 150 μm orsmaller be 100% by weight. If the proportion of secondary particles of aparticle size of 150 μm or smaller is less than 80% by weight, therubber particles are not sufficiently dispersed when melt kneaded in akneading machine. If the proportion of secondary particles of a particlesize of 250 μm or smaller is less than 97% by weight, narrow parts ofgold wires and lead pins will be clogged with the particles to causedeformation or cut of the gold wires, and deformation of lead pins. Thesecondary particles are observable as black spots in a molded resin in,for example, an X-ray photograph or an ultrasonic micrograph.

Component (C) is preferably used in an amount of 0.1 to 4.0% by weight,particularly 0.1 to 2% by weight, based on the total epoxy resincomposition. In amounts less than 0.1% by weight, the butadiene rubberparticles tend to fail to produce sufficient effects in reducing thestress. In amounts exceeding 4.0% by weight, the reliability of thesemiconductor element tends to be impaired due to the ionic impuritiescontained in the rubber particles, and the rubber particles tend to failto be dispersed with sufficient uniformity.

The epoxy resin composition comprising components (A) to (C) can furthercomprise (D) silicone oil having at least one amino group per molecule.Addition of the specific silicone oil brings about further improvementon reliability against moisture. This seems to be because the siliconeoil acts like a surface treating agent to improve adhesion between theepoxy resin composition and a semiconductor element and also because thewater repellency of the silicone oil contributes to moisture resistance.Preferred silicone oil as component (D) includes one represented byformula (I):

wherein two R's, which may be the same or different, each represent amonovalent organic group having an amino group; and n represents aninteger of 0 to 40.

The silicone oil typically exemplified by the one represented by formula(I) is prepared in a known manner. It is preferably used in an amount of0.01 to 1.0% by weight, particularly 0.01 to 0.3% by weight, based onthe total epoxy resin composition. Sufficient effects on reliabilityagainst moisture are not produced with an amount less than 0.01% byweight. Addition of more than 1.0% by weight of the silicone oil tendsto reduce the molding properties of the epoxy resin composition. Thesilicone oil to be used usually has a molecular weight of about 300 to2000. In using the silicone oil of formula (I), anaminopropyl-terminated dimethylsiloxane having the n number of 0 to 40is usually used, in which the n number is preferably 0 to 20. In formula(I), the amino-containing monovalent organic group R is preferably anamino-containing alkyl group having 1 to 8 carbon atoms. Of thesepreferred aminoalkyl-terminated dimethylsiloxane compounds,aminopropyl-terminated dimethylsiloxanes represented by formula (II) arestill preferred.

wherein n is 0 to 20.

In addition to components (A) to (D), the epoxy resin composition canfurther contain, if desired, various appropriate additives, such as cureaccelerators, inorganic fillers, halogen-containing flame retardants(e.g., brominated novolak epoxy resin) and assistants therefor (e.g.,antimony trioxide), pigments (e.g., carbon black), and silane couplingagents (e.g., γ-glycidoxy-propyl-trimethoxysilane,γ-mercaptopropyl-tri-methoxysilane, andγ-aminoethyl-aminopropyl-trimethoxysilane).

The cure accelerators include amine type cure accelerators andphosphorus type cure accelerators. Useful amine type cure acceleratorsinclude imidazoles (e.g., 2-methyl imidazole) and tertiaryamines (e.g.,triethanol-amine and diazabicyclo-undecene). Useful phosphorus type cureaccelerators include triphenylphosphine. These cure accelerators can beadded either individually or as a combination thereof. The cureaccelerators is preferably added in an amount of 0.1 to 1.0% by weightbased on the total epoxy resin composition. Flowability of the resincomposition being taken into consideration, a still preferred amount is0.15 to 0.35% by weight.

The inorganic fillers that can be added to the composition are notparticularly limited, and customarily employed ones, such as quartzglass powder, silica powder, alumina and talc, can be used. Sphericalfused silica powder is particularly preferred. The inorganic filler ispreferably used in an amount of 70 to 95% by weight based on the totalepoxy resin composition.

The epoxy resin composition of the invention can be prepared, forexample, as follows. The essential components (A) to (C), optionalcomponent (D), and necessary additives are compounded all at once bymelt kneading under heat in a kneading machine, such as a mixing roll.After cooling the molten mixture to room temperature, the mixture isground in a conventional manner. If desired, the resulting powder ispressed into tablets.

The butadiene rubber particles (component (C)) do not greatly changetheir secondary particle sizes while being kneaded in so far as anordinary mixing method is used. It is believed that the secondaryparticle size does not undergo great change either while the epoxy resincomposition is molded and cured.

The epoxy resin composition of the invention can also be prepared bypreliminarily melt mixing component (C) with the whole or part ofcomponent (A) or (B) and further melt mixing the other components intothe molten mixture, followed by cooling, grinding and, if desired,pressing into tablets.

Encapsulation of a semiconductor element in the epoxy resin compositionof the invention is not particularly limited and can be carried out in aknown molding method, typically transfer molding.

The semiconductor device thus obtained is excellent in low stressproperties owing to the butadiene rubber particles having specificsecondary particle characteristics which are uniformly dispersed in theepoxy resin composition.

The present invention will now be illustrated in greater detail withreference to Examples and Comparative Examples, but it should beunderstood that the invention is not deemed to be limited thereto.Unless otherwise noted, all the percents are given by weight. Materialsused in Examples and Comparative Examples are as follows.

Epoxy resin:

o-Cresol novolak epoxy resin (epoxy equivalent: 200; softening point:85° C.)

Phenolic resin A:

Phenol novolak resin (hydroxyl equivalent: 110; softening point: 80° C.)

Phenolic resin B:

Phenol aralkyl resin represented by formula (III) (hydroxyl equivalent:170; softening point: 80° C.)

 Wherein n is 0 to 10.

Brominated epoxy resin (flame retardant):

Brominated novolak epoxy resin (epoxy equivalent: 275; softening point:85° C.)

Flame retardant assistant:

Antimony trioxide

Silicone oil:

Aminopropyl-terminated dimethylsiloxane represented by formula (IV):

 wherein n is 4 to 8.

Release agent:

Carnauba wax

Cure accelerator:

1,8-Diazabicyclo[5.4.0]undecene-7 (DBU)

Pigment:

Carbon black

Silica powder:

Spherical fused silica powder (average particle size: 25 μm)

Silane coupling agent A:

γ-Glycidoxy-propyl-trimethoxysilane

Silane coupling agent B:

γ-Mercaptopropyl-trimethoxysilane

Butadiene rubber particles c1:

Methyl methacrylate-butadiene-styrene copolymer (primary particle size:0.2 μm; average secondary particle size: 150 μm; secondary particles of150 μm or smaller: 50%; secondary particles greater than 250 μm: 25%;secondary particles of 500 μm or greater: 15%; butadiene content: about50%; methyl methacrylate content: about 20%)

Butadiene rubber particles c2 to c6:

Butadiene rubber particles c1 were freeze-ground using liquid nitrogenand classified. Grinds of different sizes were appropriately mixed up toprepare butadiene rubber particles c2 to c6 shown in Table 1 below.

TABLE 1 Butadiene Rubber Particles c1 c2 c3 c4 c5 c6 Avg. sec. particle150 20 50 70 70 70 size (μm) Ratio of sec. 50 100 90 80 80 70 particlesof 150 μm or smaller (wt %) Ratio of sec. 75 100 100 100 95 100particles of 250 μm or smaller (wt %) Ratio of sec. 25 0 0 0 5 0particles greater than 250 μm (wt %) Ratio of sec. 15 0 0 0 0 0particles of 500 μm or greater (wt %)

EXAMPLES 1 TO 11 AND COMPARATIVE EXAMPLES 1 TO 7

Materials shown in Tables 2 and 3 below were compounded all at once andmelt kneaded in a roll mill heated at 100° to 120° C. for 5 minutes.After cooling, the compound was ground to powder and pressed intotablets to obtain an epoxy resin composition for semiconductorencapsulation.

TABLE 2 Example No. Comparative Example No. Composition (wt %) 1 2 3 4 56 7 8 9 10 11 1 2 3 4 5 6 7 Epoxy resin 100 100 100 100 100 100 100 8090 80 80 100 100 100 100 100 100 100 Phenolic resin A 60 60 60 60 60 6060 — 30 — — 60 60 60 60 60 60 60 B — — — — — — — 80 40 80 80 — — — — — —— Brominated epoxy 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10resin Flame retardant 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 assistantButadiene c1 — — — — — — — — — — — 7 — — — — — 10 Rubber c2 7 7 7 — — —— 7 7 7 7 — — — — — — — Particles c3 — — — 7 — 4 10 — — — — — — — — — —— c4 — — — — 7 — — — — — — — — — — — — — c5 — — — — — — — — — — — — — —7 — 10 — c6 — — — — — — — — — — — — — 7 — 4 — Silicone oil — 2 2 — 2 1 22 2 0.1 6 — — 2 — 2 1 2 Release agent 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 55 Cure accelerator 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Pigment 4 4 4 4 44 4 4 4 4 4 4 4 4 4 4 4 4 Silica powder 500 500 500 500 500 500 500 500500 500 500 500 500 500 500 500 500 500 Silane coupling A 3 3 — 3 3 — —3 — 3 3 3 3 — 3 — 3 — agent B — — — — — — — — 3 — — — — — — — — —

The epoxy resin compositions prepared in Examples and ComparativeExamples were evaluated as follows. The results obtained are shown inTables 3 and 4 below.

1) Number of Agglomerates

Tablets were transfer molded (175° C.×2 min.) to prepare a 28-pin smalloutline J lead package (SOJ-28p) for a semiconductor element. The numberof black spots, which are agglomerates of rubber particles, in thepackage was counted with a scanning acoustic tomograph.

2) Reliability against Moisture

A semiconductor element (3 mm×5 mm) having aluminum electrodes depositedthereon was mounted on a metal frame for a 16-pin dual inline package(DIP-16). After wire bonding, the chip was sealed in the epoxy resincomposition. The resulting semiconductor devices were subjected to aninitial electrical test. Those accepted in the test were put in apressure cooker kept at 125° C. and 85% RH, and a bias voltage of 30 Vwas applied (PCB test), and the resistivity was measured at regular timeintervals. The devices whose resistivity increased twice or more in thePCB test were rejected, and the time when half of the tested deviceswere rejected was recorded.

3) Linear Expansion Coefficient, Flexural Modulus, and Stress Index

Test specimens were molded from the epoxy resin composition, and alinear expansion coefficient (α) and a flexural modulus (E) weremeasured with a thermomechanical analyzer (MJ-800GM, manufactured byRigakusha) and a bending tester (Autograph AG-500C, manufactured byShimadzu Corp.), respectively. The product of the linear expansioncoefficient (α) and the flexural modulus (E), α×E, was obtained as astress index.

TABLE 3 Example No. 1 2 3 4 5 6 7 8 9 10 11 Number of agglomerates 0 2 35 0 1 8 4 3 3 3 of rubber particles per package Average life in PCB test550 700 700 550 600 600 600 700 650 700 710 (hr) Flexural modulus (E)1300 1320 1320 1350 1300 1380 1250 1250 1320 1350 1100 (kg/mm²) Linearexpansion 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 coefficient (α); ×10⁻⁶ (1/° C.) Stress index (α × E) 2340 2376 2376 2430 2340 2484 22502250 2376 2430 1980

TABLE 4 Comparative Example No. 1 2 3 4 5 6 7 Number of <50 — 30 <50 18<50 <50 agglomerates of rubber particles per package Average life 450450 500 400 500 500 500 in PCB test (hr) Flexural 1350 1590 1350 13501400 1280 1300 modulus (E) (kg/mm²) Linear 1.8 2.0 1.8 1.9 1.8 1.9 1.9expansion coefficient (α); × 10⁻⁶ (1/° C.) Stress index 2430 3180 24302565 2520 2430 2470 (α × E)

As can be seen from the results in Tables 3 and 4, more agglomerates ofrubber particles, seen as black spots, were observed in the cured resinsof Comparative Examples than in those of Examples, and the samples ofComparative Examples had shorter average life in the PCB test than thesamples of Examples, indicating poorer reliability against moistureattack. Compared with the comparative samples, the samples of Exampleshad a longer average life and lower values in both linear expansioncoefficient and stress index.

As described above, the present invention can obtain an epoxy resincomposition for semiconductor encapsulation by containing butadienerubber particles (component C) together with the epoxy resin (componentA) and phenolic resin (component B). Thus, despite of containing thebutadiene rubber particles (component C) together with other components,since the butadiene rubber particles comprise the above-describedspecific secondary particles, the butadiene rubber particles (componentC) in the epoxy resin composition for semiconductor encapsulation of thepresent invention are uniformly dispersed in the epoxy resincomposition. As a result, butadiene rubber particles do not agglomerate,so that rubber particles having a large particle sized are not formed,and it is also not necessary to pass complicated steps. This makes itpossible to easily obtain the epoxy resin composition for semiconductorencapsulation at low cost. Further, the semiconductor device of thepresent invention is prepared by encapsulating a semiconductor elementusing a specific epoxy resin composition having the butadiene rubberparticles (component C) uniformly dispersed therein. Therefore, in thesemiconductor device of the present invention, the butadiene rubberparticles (component C) comprising the specific secondary particles areuniformly dispersed in the encapsulating resin, so that thesemiconductor device is provides with excellent low stress properties.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. An epoxy resin composition for semiconductorencapsulation, comprising: (A) an epoxy resin, (B) a phenolic resin, and(C) butadiene rubber particles having an average particle size ofsecondary particles of 100 μm or smaller and having such a sizedistribution that the proportion of secondary particles having aparticle size of 250 μm or smaller is 97% by weight or more, and theproportion of secondary particles having a particle size of 150 μm orsmaller is 80% by weight or more.
 2. The epoxy resin composition asclaimed in claim 1, wherein said butadiene rubber particles as component(C) is present in an amount of 0.1 to 4.0% by weight based on the totalcomposition.
 3. The epoxy resin composition as claimed in claim 1,wherein said butadiene rubber particles as component (C) is a methylmethacrylate-butadiene-styrene copolymer.
 4. The epoxy resin compositionas claimed in claim 1, wherein said butadiene rubber particles ascomponent (C) has a core-shell structure, wherein the core portioncomprises a styrene-butadiene copolymer, and the shell portion comprisesmethyl methacrylate or a methyl methacrylate-styrene copolymer.